1. Field of the Invention
The present invention relates to an exposure method and a projection exposure apparatus used in a lithography process in production of semiconductor elements, liquid crystal elements, or the like.
2. Related Background Art
There are two fundamental exposure methods conventionally used in such projection exposure apparatus. In one method, a photosensitive substrate such as semiconductor wafer, glass plate, etc. is exposed to light in a step and repeat method through a projection optical system having an exposure field which can include the whole pattern of mask or reticle. The other is a scan method in which a reticle and a photosensitive substrate are opposed to each other at the both sides of a projection optical system under illumination light of arch slit illuminates the reticle, and the reticle and the photosensitive substrate are relatively scanned for exposure under the illumination. Steppers employing the former step and repeat exposure method are leading apparatus recently used in the lithography process. The step and repeat exposure method has been improved in resolution, overlay accuracy, throughput, and so on, and became superior in these respects to aligners employing the latter exposure method. Therefore, it is considered that such steppers employing the step and repeat exposure method will be leading in lithography for a while.
It is proposed for the step and repeat exposure method that the photosensitive substrate and a best imaging plane of projection optical system be relatively moved in a direction of optical axis during exposure of one shot area in order to increase an apparent depth of focus of the projection optical system. This exposure method will be hereinafter referred to as a successive focussing exposure method. In this successive focussing exposure method, the moving amount in the optical axis direction is determined considering a real depth of focus of projection optical system and micro unevenness on the photosensitive substrate. The best imaging plane of projection optical system is arranged to be located between the top and the bottom of the unevenness on the photosensitive substrate surface during the movement.
Meanwhile, a novel scan exposure method achieving high resolution has been recently proposed as a step and scan method on pp 424-433, SPIE vol. 1088, "Optical/Laser Microlithography II", 1989. The step and scan method uses both a scan method, in which a reticle is one-dimensionally scanned and a photosensitive substrate is also one-dimensionally scanned at a speed synchronized with the reticle speed, and a step method, in which the photosensitive substrate is stepped in a direction perpendicular to the scan exposure direction.
FIG. 11 is a drawing to illustrate a concept of the step and scan method. In FIG. 11, a shot area of one chip or multiple chips is scanned for exposure with illumination light RIL of arch slit in the X-direction on a photosensitive substrate or wafer W. The wafer is stepped in the Y-direction. In FIG. 11 a broken line shows a sequence of exposure of step and scan as will be hereinafter referred to as S and S, so that the S and S exposure is carried out on shot areas SA.sub.1, SA.sub.2, . . . , SA.sub.6 in this order, and then on shot areas SA.sub.7, SA.sub.8, . . . , SA.sub.12 arranged in the Y-direction in the center of the wafer. In the aligner of the S and S method as disclosed in the above-mentioned reference, an image of reticle pattern illuminated by the arch slit illumination light RIL is focussed on the wafer W through a one-quarter reduction projection optical system. Thus, a scan speed of reticle stage in the X-direction is controlled precisely to four times of that of wafer stage in the X-direction. The arch slit illumination light RIL is used because a demagnification system with a combination of refraction and reflection elements is employed as the projection optical system, and advantage is taken of various abberations being zero in a narrow annular region apart at a certain distance from the optical axis. An example of such reflection reduction projection system is disclosed in U.S. Pat. No. 4,747,678.
However, it is impossible that the successive focussing exposure method for the step and repeat method is applied to the step and scan method. In detail, the step and repeat method takes such a structure that the reticle/wafer and the illumination optical flux/exposure flux cannot be moved relative to each other in a direction perpendicular to the optical axis of the projection optical system, i.e., in a direction of wafer plane, upon exposure of one shot area. Therefore, a point of pattern in a transfer region on a reticle may be exposed at a plurality of focus points by relatively moving the wafer and the projection optical system in a direction of the optical axis upon exposure. In contrast, the step and scan method takes such a structure that the reticle/wafer and the illumination flux/exposure flux may be moved relative to each other in the direction perpendicular to the optical axis upon exposure of one shot area. In this structure, if the wafer and the projection optical system are moved relative to each other in the optical axis direction upon exposure, there would be mixed focussed parts and unfoccused parts on the wafer depending on positions in the transfer region on the reticle. Accordingly, if the same successive focussing method as in the step and repeat method is used for the S and S method, an increase in a depth of focus could not be expected, but degrading the resolution of image on the contrary.